Volume Gods rays are not well defined
Because your light source is too big.

God rays are just interesting/complicated shadow patterns hanging in space. If your light is throwing soft, boring shadows, the god rays of those shadows hanging in the air area going to be blurred and boring too.

Just as a start, here's your scene with the Icosphere mesh light at 1/20th the size, and 100x the power:



Since we just need a small sphere light for this, we'll replace that mesh with a point light as it's a bit more efficient and easier to control. I've set this one to size=0.02 (2cm) and the power to 1000. I also got rid of the volume cube around the object, unless you want some 2-tier rays, you don't need that AND the world volume.

Looks like this


You can vary the sharpness of the shadows by adjusting the lamp size. That will sharpen the rays as well. Here's the point light at 5mm instead of 2cm



Or tweak the lamp colors. Here's 2cm point lamp with an orange color. I also switched to branched path tracing, as it tends to be good with "volume show" scenes like this since you can turn up volume sampling. Plus I flipped on the denoiser: